Thermal cleavage (cracking) of hydrocarbons is usually carried out in pyrolytic furnaces. A conventional pyrolytic furnace includes a radiation zone heated with burners. Thermal cracking of the hydrocarbons is performed in cracking tubes in the radiation zone. Cracking of the hydrocarbons can be facilitated by preheating the hydrocarbons in a convection zone of the pyrolytic furnace before they are introduced into the radiation zone.
Among the large number of prior art pyrolytic furnaces, the foregoing has proven itself especially well. FIG. 1 illustrates a prior art arrangement of the cracking tubes in the radiation zone of one such pyrolytic furnace. In the version shown, sixteen cracking tubes are combined into groups A, B, and C. The regions of the individual groups are indicated by dot-dash lines. The hydrocarbons to be cracked are conducted in the direction of the arrows from the top into the cracking tubes in the radiation zone. The cracking tubes extend parallel and linearly (Z direction) to the bottom region of the radiation zone, where two cracking tubes are united via a collective tube section. Tube elbows adjoin the resultant eight cracking tubes of the group. From the tube elbows, straight tube sections parallel, vertically arranged (Z direction) lead to the upper region of the radiation zone. At that location, respectively, four tube sections are combined in collective tube sections into two cracking tubes. These two cracking tubes are united in a further manifold tube section to the outlet tube of the respective group. The cracked hydrocarbons are conducted out of the radiation zone of the pyrolytic furnace through the three outlet tubes of the groups. The linear tube sections and the collecting tube sections of the individual groups are arranged in one row. They all lie in the same plane (X/Z plane). Only the tube elbows in the bottom region of the radiation zone exhibit projections and jut out of this plane.
The cracking tubes are customarily suspended in the radiation zone of the pyrolytic furnace. Longitudinal expansions caused by temperature variations require a suspending of the cracking tubes. However, the mechanical stress on the cracking tubes due to their inherent weight is considerable in this case. For this reason, stress overloads occur especially in the tube elbows, particularly at high temperatures.